[3.14] Add a What's New entry for Emscripten (GH-137035) (#137192)

Add a What's New entry for Emscripten (GH-137035)
(cherry picked from commit d7e12a362a2a4a4b7d52a343ab5940be2cbcc909)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>